Genetic Disorders & Sex Linked Traits

1. Genetic Disorders&Sex Linked Traits Honors Biology Unit 5 2012-2013

2. Recessive Disorders • Disorders that are only expressed in the phenotype when 2 recessive alleles are present. • DD = Normal • Dd = Carrier • dd = Affected by disorder

3. 2 Examples of Recessive Disorders

4. Tay-Sachs Disease • A genetically caused disease in which the gene to make the enzyme Hex-A (Hexosaminidase A) is not working. • Hex-A is an enzyme that breaks down the lipid GM2 ganglioside. • Without Hex-A this lipid accumulates on nerve cells, specifically in the brain causing severe brain damage. • Victims of this disease to not live past age 5

5. Common in Eastern European Ashkenazi Jews • This is a group of people descendent of medieval Jews from the Rhineland area. (Rhineland: near the river Rhine in Germany) • Common in this population (1 in 30)

6. Tay-Sachs Brain

7. How is Tay-Sachs disease passed? Each parent must be a Carrier Offspring: 25% Normal 50% Carriers 25% Tay-Sachs

8. Cystic Fibrosis • Thick mucus is produced by the body • Mucus fills lungs causing lung infections • Mucus blocks pancreas which causes digestive problems • Mucus can block bile ducts in liver causing liver failure.

9. Cystic Fibrosis

10. Cystic Fibrosis Most common in Caucasian (white) populations (1 in 2500 to 3500) 1 in 17,000 African Americans 1 in 31,000 Asian Americans

11. Carriers of Cystic Fibrosis Offspring: 25% Normal 50% Carriers 25% Cystic Fibrosis

13. Dominant Disorders Disorders that are expressed in the phenotype of heterozygous and homozygous dominant individuals • DD = Affected • Dd = Affected • dd = Normal

14. 2 Examples

15. 1) Huntington’s Disease • Brain cells degenerate over time • Mood swings, loss of muscle control, loss of memory and inability to learn, death. • Usually adult-onset, appears around ages 40-50 • Outlook is 10-15 years of survivablity • hh = Normal • HH or Hh = will get and die of this disease

16. Person with Huntington’s and a person without Huntington’s Hh x hh Offspring: 50% Huntington’s 50% Normal

17. Huntington’s is most common in certain parts of Venezuela (700 in 100,000)Generally affect 3-7 in 100,000 of European ancestryLess common in African-American & Asian American

18. 2) Marfan’s Syndrome • Defective gene for fibrillin-1 that results in abnormal connective tissue • Aorta may stretch or become weak, causing aortic rupture, the leading cause of death • Eye/lens problems • Excessive long bone growth (long arms & fingers) • Hypermobile joints (too flexible)

19. Chromosome Structure Variations • Chromosomes can be broken by X-rays and by certain chemicals. The broken ends spontaneously rejoin, but if there are multiple breaks, the ends join at random. This leads to alterations in chromosome structure. • Breaking the chromosome often means breaking a gene. Since most genes are necessary for life, many chromosome breaks are lethal or cause serious defects. • Also, chromosomes with structural variations often have trouble going through meiosis, giving embryos with missing or extra large regions of the chromosomes.

20. Chromosome Structure Variations • The major categories: duplication(an extra copy of a region of chromosome) deletion(missing a region of chromosome) inversion(part of the chromosome is inserted backwards) translocation(two different chromosomes switch pieces).

21. Chromosome Variations • Changes in number and structure are possible: first look at number variations. • Aneuploidy: having an extra or missing chromosome– is fairly common in sperm and eggs. Errors in meiosis causes chromosomes to not separate equally into the gametes. The rate of aneuploidy in males is constant: 1-2% of sperm have an extra or missing chromosome.

22. Chromosome Variations • In females, the rate increases with age. This is illustrated by the frequency of Down syndrome births at different ages of mother. Down syndrome is the most frequent result of aneuploidy.

23. Chromosome Number Variations:Aneuploidy • Except for the X and Y, humans don’t survive with only 1 copy of any chromosome. Also, 3 copies is lethal in most cases. Aneuploidy is a major cause of spontaneous abortion in early pregnancy. • Down Syndrome is the most common human aneuploidy. It is also called trisomy-21, meaning 3 copies of chromosome number 21.

24. *Having 3 chromosomes of each kind instead of 2*Normally trisomy results in death. Trisomy: 1 Example

25. Down Syndrome: • a genetic condition in which the individual has 3 copies of the 21st chromosome. • Genotype: 3 copies of 21st chromosome

26. Down Syndrome: • Phenotype: People with Down’s have a characteristic appearance: flattened face, turned up nose, epicanthal folds at the outer corners of the eyes. In most cases the diagnosis is made immediately at birth. Heart defects, protruding tongue, and mental retardation are also found in most people with Down’s. Occurs about 1 in 1000 births.

28. Sex Linked Traits

29. Sex Linked Traits • Traits that occur on the X or Y chromosome • X-linked traits are traits found on the X Chromosome

30. Sex-linked Genes • Genes on the X chromosome are called “sex-linked”, because they expressed more often in males than in females • There are very few genes on the Y chromosome. • Since males only have one X chromosome, all genes on it, whether dominant or recessive, are expressed.

31. Sex-linked Genes • In contrast, a mutant gene on an X chromosome in a female is usually covered up by the normal allele on the other X. Most mutations are recessive. So, most people with sex-linked genetic conditions are male. • Another fact about sex-linked genes. Males produce ½ their sperm with their X chromosome, and half with their Y chromosome. The X-bearing sperm lead to daughters and the Y-bearing sperm lead to sons. So, sons get their only X from their mothers, and the father’s X goes only to daughters. • The Y chromosome is passed from father to son.

32. Why can females have 2 copies of the X chromosome, when 2 copies of most chromosomes is deadly? • Answer: In each cell one of the X chromosomes ‘turns off’. This turned off chromosome is known as a Barr body. The effect of Barr bodies can be seen in Calico colored cats.

33. Why can females have 2 copies of the X chromosome, when 2 copies of most chromosomes is deadly? • Example: Calico Colored cats. A calico cat has patches of orange and patches of black X = orange X1 = black Males: XY = orange / X1Y = black Females: XX = orange / X1X1 = black / XX1 = calico Because individual cells determine which X chromosome to deactivate, the patches of color arise

34. Colorblindness • We have 3 color receptors in the retinas of our eyes. They respond best to red, green, and blue light. • Each receptor is made by a gene. The blue receptor is on an autosome, while the red and green receptors are on the X chromosome (sex-linked).

35. Colorblindness • Most colorblind people are males, who have mutated, inactive versions of either the red or the green (sometimes both) color receptors. Most females with a mutant receptor gene are heterozygous: the normal version of the receptor genes gives them normal color vision.

36. Colorblind Test! • You will see circles with many colors of dots • The dot pattern makes up a number • What number do you see?

37. With Color Vision:

38. This one you can even see in black and white

39. Color Blind Test What number do you see?

40. Color Blind Test What number do you see?

41. This what you would see if you were color blind What number do you see?

42. Color Blind Test What number do you see?

43. Color Blind Test What number do you see?

44. Color Blind Test What number do you see?

45. Color Blind Test What number do you see?

46. With color vision you see this: But if you were red-green colorblind…. You would see the #: 5

47. What do the colorblind see? Types of Colorblindness

48. Types of Colorblindness – Normal No color vision Protanopia: no red Deuteranopia: no green Tritanopia: no blue

49. How to write Alleles for X-Linked Traits • Women: • Normal: XBXB • Carrier: XBXb • Colorblind: XbXb • Men: • Normal: XBY • Colorblind: XbY

50. Hemophilia • Hemophilia is a disease in which the blood does not clot when exposed to air. People with hemophilia can easily bleed to death from very minor wounds. Hemophilia is another sex-linked trait. • Hemophilia is treated by injecting the proper clotting proteins, isolated from the blood of normal people. In the early 1980’s, the blood supply was contaminated by HIV, the AIDS virus, and many hemophiliacs contracted AIDS at that time. Small cuts, scrapes and bruises can be life threatening 1 in 10, 000 males 1 in 100,000,000 females